Effects of Indium on Wetting and Interfacial Features of a Sn-40Bi Alloy in a Copper Substrate

The influence of In on the solidification path, microstructure, wettability, tensile properties, and interfacial features of a Sn-40Bi alloy was studied using Computer Coupling of Phase Diagrams and Thermochemistry (CALPHAD) calculations, differential scanning calorimetry, scanning electron microscopy, x-ray diffraction, wettability testing and tensile testing. The microstructures were composed of Sn-rich matrixes with Sn-Bi and BiIn-Bi constituents. Significant sensitivity of the microstructural scale to the solidification cooling rate was also demonstrated by employing different processing conditions. Because of the relatively high BiIn fraction on the microstructure, a limited elongation of approximately 7% was achieved, with ultimate tensile strength of 65 MPa. Cu-6(Sn, In)(5) was the intermetallic phase of the reaction film for the Sn-Bi-In/Cu interface under all tested conditions, including as-soldered and aged at 100 degrees C and 120 degrees C for 120 h, 240 h, and 360 h. Wetting angles for the ternary Sn-Bi-In alloy were maintained at approximately 20 degrees, which is smaller than usual for Sn-Bi alloys. The new phase, Cu-6(Sn, In)(5), produced instead of Cu6Sn5, showed a lower growth rate than that observed in In-free couples. The mechanisms of wetting and the Cu-6(Sn, In)(5)-layer growth are discussed.